In manufacturing integrated circuits, it is common to form a dielectric layer on the surface of a silicon wafer and then pattern the wafer by removing portions of the dielectric layer. One widely used dielectric layer is silicon dioxide. Silicon dioxide will also form on the surface of a silicon substrate that is exposed to air. This is typically referred to as a "native" SiO.sub.2 layer. Native oxides are undesirable as they inhibit subsequent processing of the wafer and therefore must be removed. One way to pattern silicon dioxide and to remove native oxides is by wet cleaning (etching). As explained in the book entitled Semiconductor Integrated Circuit Processing Technology by W. R. Runyan and K. E. Bean, Copyright 1990 by Addison-Wesley Publishing Company Inc., on pages 264-267, hydrofluoric acid (HF) in various dilutions in water and often buffered with ammonium fluoride (NH.sub.4 F) is the standard silicon dioxide wet etchant. (The buffer ratio for oxide etching with HF generally means the volume ratio of a solution of 40% by weight NH.sub.4 F in water to 49% HF.) In dilute water solutions, HF will partially dissociate and give EQU HF.fwdarw.F.sup.- +HF.sub.2.sup.-
Much research indicates that HF.sub.2.sup.- species is responsible for etching silicon dioxide. See H. Kikuyama, N. Miki, K. Saka, J. Takano, I. Kawanabe, M. Miyashita and T. Ohmi, Principles of Wet Chemical Processing in ULSI Microfabrication, IEEE Transactions on Semiconductor Manufacturing, Vol. 4, No. 1, February 1991, pp. 26-35. See also H. Proksche, G. Nagorsen and D. Ross, The Influence of NH.sub.4 F on the Etch Rates of Undoped SiO.sub.2 in Buffered Oxide Etch, Journal of the Electrochemical Society, Vol. 139, No. 2, February 1992, pp. 521-524.
While having advantages as an etchant, hydrofluoric acid is a highly toxic and corrosive chemical, posing severe safety and health constraints, especially for uses which employ HF at elevated temperatures. Toxic HF vapors at room temperatures pose health risk; toxic HF vapors at high temperatures also promote corrosion of instruments in addition to the health risk. Since buffered hydrofluoric acid (BHF) contains only a very small amount of hydrofluoric acid, the health concerns associated with it are lesser, but the large amount of fluorine containing species generated in the formulation is an environmental risk.
U.S. Pat. No. 5,268,069 issued Dec. 7, 1993 to Chapple-Sokol et al. entitled "Safe Method For Etching Silicon Dioxide" teaches using anhydrous ammonium fluoride (NH.sub.4 F) as a source of hydrofluoric acid. Specifically, the patent proposes heating the anhydrous ammonium fluoride to above its sublimation temperature of 100.degree. C. to formulate the reaction: EQU NH.sub.4 F.sub.(c) .fwdarw.HF.sub.(g) +NH.sub.3(g)
Hence, Chapple-Sokol et al.'s method ultimately produces the same species as hydrofluoric acid produces in aqueous medium, since it utilizes controlled amounts of water vapor to mix with the ammonium fluoride vapors. Their method is safer than traditional hydrofluoric acid etch since the starting material is less toxic and easier to handle. However, Chapple-Sokol et al.'s method is disadvantageous due to the elevated heating temperature necessary to formulate the reaction and due to the time and expense necessary to heat the anhydrous ammonium fluoride. What is needed is an oxide etch chemistry which has safety benefits over the traditional HF wet etch and also has economic and time benefits over U.S. Pat. No. 5,268,069 to Chapple-Sokol et al.
It is accordingly an object of this invention to provide an etch chemistry for silicon oxide that is both benign and easy to use.
Other objects and advantages of the invention will be apparent to those of ordinary skill in the art having the benefit of the following description and drawing figures.